The invention relates to an apparatus for the continuous recycling of synthetic plastics material, preferably polyester, comprising a first receptacle for the material, in which receptacle movable tools are disposed which comminute and/or mix the material, and from which the so-processed material is transported through a vacuum-tight closable conduit into a second receptacle, to the interior of which an evacuating device is connected and in which also movable tools are provided for processing the material, wherein that material is conveyed off the second receptacle by means of a screw.
An apparatus of initially described kind is known (AT 396,900 B). In this apparatus, the material is comminuted and mixed within the first receptacle and is pressed by the circulating tools into the conduit leading to the second receptacle. The second receptacle is evacuated in order to protect the material that is sensitive against air contact when heated and in order to decrease the drying time. The pre-processed synthetic plastics material flowing within the channel towards the second receptacle acts in tightening manner but cannot completely prevent air streaming from the first receptacle into the second receptacle. A completely tight closure of the second receptacle would be possible with a closure disposed within the channel, but then a continuous operation of the apparatus is no longer possible.
It is an object of the invention to improve an apparatus of the initially described kind so that a continuous operation is possible while assuring a sufficient vacuum-tightness of the second receptacle. Thereby, the processed material should be treated gently; in particular a degradation of the chains of the molecules should be prevented as far as possible. The invention solves this task in that a screw is bearingly supported within the conduit, the volutions of this screw conveying the material from the first receptacle into the second receptacle, wherein between the screw volutions and the second receptacle at least one damming element is provided within the conduit and disposed within the flow of the conveyed material. This damming element results in an intensive compression zone for the plastic material that is sufficiently vacuum-tight to permit the continuous operation of the apparatus.
It is known (AT E 128,898 T) to dispose a screw in a channel connecting two chambers disposed one behind the other, when seen in flow direction of the processed material. However, within this apparatus, the second chamber is not evacuated and the apparatus does not operate continuously because the screw is rotated only when in the first chamber predetermined conditions of the material are reached. Therefore, with such an apparatus, material that is sensitive against air, in particular polyester, cannot be treated successfully.
Within the spirit of the invention, important advantages are obtained if the screw is an agglomerating screw, because in that way highly sensitive agglomerated synthetic plastics material, in particular polyester, of good quality can be obtained. When compared with apparatus plasticizing the material, with an agglomerating apparatus only so much power is introduced into the processed synthetic plastics material, which mostly is introduced into the intake opening of the housing of the agglomerating screw in a pre-comminuted form, so that the particles of the synthetic plastics material are only slightly caked, but are not plasticized. Therefore, the particles of the synthetic plastics material adhere to each other or the synthetic plastics material is sintered, resulting in material that is as loose as possible when it is supplied to the second receptacle.
As it is known, within an agglomerating screw the ratio of effective screw length to the nominal diameter of the screw is less than 16. Within the spirit of the invention particular favorable values for this ratio are between 4.5 and 12. By xe2x80x9ceffective lengthxe2x80x9d of the agglomerating screw, that distance is to be understood which is between the downstream end of the screw and that margin of the intake opening through which the material is supplied into the housing of the screw. Further, within an agglomerating screw, the screw volutions in the delivery zone (that is, the zone neighboring the exit opening for the material) are cut deeper than within a plasticizing screw. Within the latter, the ratio between the depth of the screw volutions and the screw outer diameter usually amounts to about 0.05 to 0.08, whereas within an agglomerating screw this ratio, as a rule, amounts to about 0.1 to 0.12.
Within the spirit of the invention, a damming element can be constructed in different manners: According to a preferred embodiment of the invention, the damming element is formed by screw volutions, the direction of their threads being opposite to those of the volutions of the screw. These inversely directed volutions, therefore, have a conveyance effect that acts opposite to that of the screw but is overcome by the latter. In such a manner, the processed material is dammed up in the zone of this reverse screw. The above-mentioned effect can be secured in a simple manner by the feature thatxe2x80x94when measured in axial direction of the conduitxe2x80x94the screw is longer than the oppositely directed volutions and/or that the volutions of the screw are cut deeper than the oppositely directed volutions.
Another particularly favorable construction for the damming element of the invention is that the damming element is defined by a member having a rib that is disposed cross-wise, in particular perpendicularly, to the flow direction of the material. Such a member acts in a similar manner damming on the material conveyed by the screw, in particular, if two or more ribs are provided. In order to obtain favorable flow conditions, it is suitable to provide at least two ribs having a rounded profile and disposed one behind the other, when seen in flow direction of the material.
The reverse screw can be rotated by a separate drive means so that the desired conveyance direction (opposite to that of the screw) is obtained. However, if within the spirit of the invention the damming element is connected for common rotation with the screw, such a separate drive means can be saved, if the volutions of the oppositely acting reverse screw are correspondingly oriented. A damming member having ribs can be disposed stationarily within the conduit; however it is preferred that such a damming element is also rotated, in particular by connecting it for common rotation with the screw, because in such a manner it is not necessary to abut the damming element relative to the conduit walls.
The damming element is preferably tapered at its downstream-side end in order to collect the material traces flowing along its periphery or along its volutions. If the conduit is connected eccentrically, or if desired tangentially, to the wall of the second receptacle, what is particularly suitable within the spirit of the invention, then a particular favorable embodiment consists within the spirit of the invention in that the profile of the taper of the damming element follows substantially the profile of the wall of the second receptacle in the zone of the connection of the conduit. This ensures conditions as smooth as can be at the inner wall of the second receptacle in which the material is circulated by the circulating tools. The so-created mixing zone is not impaired by this construction.
For assembling and service reasons it is suitable to dispose the damming element within a housing member forming a section of the channel, that housing member being detachably, but tightly, connected to the housing of the screw. The tight connection is necessary in order to take up the high pressures occurring in the zone of the conduit, without any loss, which pressures can reach 300 bar and more. If the damming element is constituted by a member carrying ribs, then it is suitable within the spirit of the invention to cut the volutions of the agglomerating screws deeper than the smallest dimension of the gap occurring between the periphery of the ribs of the damming element and the inner wall of the conduit, when measured in radial direction. As it can be seen, the damming element must not be formed radially symmetric to the axis of the conduit; on the contrary, it may have also the form of a toothed wheel, most of the material passing the damming element through its tooth spaces.
As already mentioned, some of the materials to be recycled, in particular polyester, are very sensitive to humidity when heated. In order to avoid that such material is adversely affected by its processing within the first receptacle, within the spirit of the invention the first receptacle is also closed vacuum-tightly and is connected to an evacuating device. The tight connection of the filling opening of this receptacle can be obtained with a sluice operating substantially continuously, for example, a cellular wheel sluice or the like, in order to enable a continuous filling of the receptacle.
As a rule, the intake opening of the housing of the screw carrying the processed material off the second receptacle is disposed at least substantially at the level of the tools circulating within the receptacle to optimally utilize the centrifugal action exerted by these tools onto the material as well as a spatula-like pressing-in of the material by the tools. Within such an embodiment, however, the invention places the mouth of the conduit in the second receptacle higher than the orbit of the tools circulating at the bottom region of the receptacle around at least one vertical axis. This avoids that material particles, which were not fully pre-processed within the first receptacle, are immediately introduced in the housing of the output screw by the circulating tools when such particles enter the second receptacle, so that such particles do not remain sufficiently long within the second receptacle. However, it is desired that the material particles remain for a predetermined time within the second receptacle by reasons of sufficient drying of humid particles or particles containing moisture. The described higher disposition of the intake opening within the second receptacle has the effect that the material particles entering the second receptacle are taken along by the material stream exerted by the tools circulating within the receptacle. This material stream has the effect that the particles rise along the receptacle wall so that particles entering this receptacle are at first taken along and moved upwardly and only thenxe2x80x94after a certain dwell timexe2x80x94can reach the zone of the intake opening of the delivery screw. This meets the requirement to keep particles that are as loose as possible within the second receptacle for as long as possible under an elevated temperature and a vacuum. The comminuting action onto the partially sintered particles entering the second receptacle is favored within the spirit of the invention if in addition to the tools circulating in the bottom zone of the second receptacle further tools, preferably coaxially disposed tools, in particular beating elements, are provided, the orbit of which is disposed above that of the tools disposed within the bottom zone of the receptacle. Suitably, the orbit of these further tools is disposed at the level of the mouth of the channel in the second receptacle.